Coal is the world's most abundant fossil fuel. However, coal has three major drawbacks: (1) Coal is a solid and is less easily handled and transported than fluidic or gaseous materials; (2) Coal contains compounds which, on burning, produce "air toxics" and the pollutants associated with acid rain; and (3) Coal is not a uniform fuel product, varying in characteristics from region to region and from mine to mine.
In fossil fuels, the ratio of hydrogen atoms to carbon atoms is most important in determining the heating value per unit weight. The higher the hydrogen content, the more liquid (or gaseous) the fuel, and the greater its heat value. Natural gas, or methane, has a hydrogen-to-carbon ratio of 4 to 1 (this is the maximum); gasoline has a ratio of almost 2.2 to 1; petroleum crude about 2.0 to 1; shale oil about 1.5 to 1; and coal about 1 to 1. Thus, if the hydrogens on half the carbons could be transferred or "rearranged" to the other half of the carbons, then the result would be half the carbons with 0 hydrogens and half with 2 hydrogens. The first portion of carbons (with 0 hydrogens) is char; the second portion of carbons (with 2 hydrogens) is a liquid product similar to a petroleum fuel oil. If this could be accomplished using only hydrogen inherent in the coal, i.e., no external hydrogen source, then the coal could be refined in the same economical manner as petroleum, yielding a slate of refined hydrocarbon products and char.
In our modern society almost every raw material is refined prior to use. Various raw ores are refined to produce useful products, such as aluminum, copper, silver, titanium, and tungsten. Except for coal, all of our fuels are refined: uranium ore, crude oil, and natural gas are refined.
Natural gas, as it comes out of the ground, contains impurities, such as CO.sub.2, heavy hydrocarbons, and sulfur containing gases. These impurities are refined out prior to use to yield predominantly a single hydrocarbon compound: methane. Natural gas represents less than 3% of the United States' known energy reserves.
Crude oil, as it comes from the ground, has limited utility. It is a dirty, sulfur-containing fuel. Hence, the petroleum industry has developed refining processes using hydrocracking techniques to produce value-added products, such as gasoline, jet fuel, and other hydrocarbon fuels and petrochemicals. Thus, gasoline refined from high sulfur crude or from light Arabian crude is still gasoline. Most of the world's crude oil reserves are remote from population centers and must be imported by industrialized nations.
Raw coal, as it comes from the ground, also has limited utility. Like its "kissing cousin", crude oil, coal contains complex hydrocarbons, sulfur, and nitrogen. High sulfur bituminous coals and high moisture subbituminous coals are very different raw materials and cannot be interchanged as fuels. Coal is our country's most abundant fossil fuel, accounting for over 95% of our fossil energy reserves. The United States has 43% more energy in coal reserves than the energy equivalent of all the oil and gas in known reserves in the whole world. Vast deposits of coal also exist in Eastern Europe, Russia, and China but are either far away from manufacturing regions or contain high levels of pollutants in proportion to the heat value of the coal.
The lignites, peats, and lower calorific value subbituminous coals have not had wide-spread use. This is due primarily to the cost of transporting a lower Btu product as well as to the danger of spontaneous combustion because of the high content of volatile matter and high percentage of moisture which is characteristic of such coals.
Since low-rank coals contain high percentages of volatile matter, the risk of spontaneous combustion is increased by dehydration, even by the non-evaporation methods. Therefore, in order to secure stability of the dehydrated coal in storage and transportation, it has been necessary to cover the coal with an atmosphere of inert gas such as nitrogen or combustion product gas, or to coat it with crude oil so as not to reduce its efficiency as a fuel. However, these methods are not economical.
The inefficient and expensive handling, transportation and storage of coal (primarily because it is a solid material) makes coal not economically exportable and the conversion of oil-fired systems to coal less economically attractive. Liquids are much more easily handled, transported, stored and fired into boilers.
In addition, coal is not a heterogeneous fuel, i.e, coal from different reserves has a wide range of characteristics. It is not, therefore, a uniform fuel of consistent quality. Coal from one region (or even of a particular mine) cannot be efficiently combusted in boilers designed for coal from another source. Boilers and pollution control equipment must either be tailored to a specific coal or configured to burn a wide variety of material with a loss in efficiency.
The transportation and non-uniformity problems are compounded by the presence of potential pollutants in raw coal. Sulfur compounds and nitrogen compounds inherent in the coal, upon combustion, create pollutants which are thought to cause acid rain. The sulfur compounds are of two types, organic and inorganic (pyritic), both of which produce SO.sub.x. The fuel bound nitrogen, i.e., organic nitrogen in the coal, combusts to form NO.sub.x. Further, because of the nonuniformity of coal, it combusts with "hot spots". Some of the nitrogen in the combustive air (air is 75% nitrogen by weight) is oxidized to produce NO.sub.x as a result of the elevated atmospheric temperatures created by these "hot spots". This so-called "thermal NO.sub.x " has heretofore only been reduced by expensive boiler modification systems.
Raw coal cleaning has heretofore been available to remove inorganic ash and pyritic sulfur but is unable to remove the organic sulfur and nitrogen compounds. Fluidized bed boilers, which require limestone as an SO.sub.x reactant, and scrubbers or NO.sub.x selective catalytic convertors (so-called combustion, and post-combustion clean air technologies) have been the conventional technologies for alleviating these pollution problems. These devices are tremendously expensive from both capital and operating standpoints, adding to the cost of power. This added power cost not only increases the cost of manufactured goods, but also ultimately diminishes domestic competitiveness with foreign goods. Further, these devices reduce power plant efficiency since they draw on power which would otherwise be available for sale. This inefficiency results in increased CO.sub.2 emissions per unit of power sold. Carbon dioxide production has been linked by some with the "greenhouse" effect, i.e. the heating of the atmosphere.
It would, therefore, be advantageous to clean up the coal by removing the organic nitrogen (fuel nitrogen), as well as the organic sulfur while providing a uniform fuel with high reactivity and lower flame temperature to reduce the thermal NO.sub.x. The coal refining process, like the petroleum refining process, employs thermal hydrocracking of the complex hydrocarbons in the raw fossil fuel to produce char (coke) and liquid product. Process gases are recycled to avoid the need for external hydrogen. Hydrocracking involves the thermal breaking or "cracking" of larger hydrocarbon molecules in the feedstock and subsequent hydrogenation of the molecular pieces using hydrogen derived from the feedstock. In coal refining, inherent moisture is another source of hydrogen. In both coal and oil refining processes, sulfur and nitrogen are removed during hydrocracking. The rearrangement of hydrogen within the coal molecule, so-called "hydrodisproportionation", produces a slate of clean, value-added co-products, just as is done with crude oil in a petroleum refining process. The coal refining process is most analogous to the commercial hydrocracking of heavy crudes, bitumen or "tar sands".
In order to overcome some of the inherent problems with coal, previous technologies have attempted to convert coal to synthetic liquid or gaseous fuels. These "synfuel" processes are capital intensive and require a great deal of externally supplied water and external hydrogen, i.e., water and hydrogen derived from other than the coal feedstock. Additionally, some of these processes produce large quantities of CO.sub.2, a "greenhouse gas". The processes are also energy intensive in that most carbon atoms in the coal matrix are converted to hydrocarbons, i.e., no char. The liquefaction of coal involves hydrogenation using external hydrogen. This differs markedly from merely "rearranging" existing hydrogen in the coal molecule as in hydrodisproportionation.